RFC3501

RFC 3501, Internet Message Access Protocol version 4 revision 1, is the IETF specification that defines IMAP4rev1, the protocol used by email clients to access messages stored on a mail server. Published in 2003, it obsoletes RFC 2060 and consolidates the IMAP4 protocol with a refined command set and response semantics.

IMAP4rev1 provides a rich set of operations to browse and manipulate mailboxes and messages. Clients can authenticate, select a mailbox, enumerate mailboxes, and create, delete, or rename mailboxes. They can fetch message data, headers, bodies, or partial content; retrieve message flags and metadata; search messages with criteria; copy and move messages between folders; and store or expunge deletions. UID-based variants ensure stable references across sessions; the CAPABILITY command allows clients to discover server features and extensions.

IMAP messages reside in mailboxes on the server and have attributes such as flags (SEEN, ANSWERED, DELETED, FLAGGED, DRAFT) and internal sequence numbers. The protocol uses tagged commands and returns data in untagged and tagged responses, following a client–server turn-taking model with defined states (Not Authenticated, Authenticated, Selected, and Logout).

Security is addressed by supporting authenticated sessions and negotiation of additional mechanisms via SASL, with encryption typically provided by TLS in practice. RFC 3501 remains a core standard for mail access, forming the basis for many IMAP extensions and implementations across servers and clients.